Hearing aids use directional microphones to improve the signal to noise ratio in locations where there are multiple sources of sound. The most useful source of sound is generally in front of the listener. If there are not many sources of sound, or if the useful source is not in front of the listener, it is advantageous to change the microphone directionality. This is accomplished in current hearing instruments by using more than one microphone. Most designs use a matched pair of omni-directional microphones. The microphones must be very closely matched, or the electronics must be able to compensate for any differences between the microphones. This is difficult to accomplish, adds to the overall expense of manufacturing, and reduces the reliability of the hearing instrument. Another approach is to use one-directional microphone and one non-directional microphones. The drawback of this approach is that three microphone openings are required on the surface of the hearing instrument.
Receiver in Canal style hearing instruments are devices which are sold in open-fitting versions for people with mild impairment. The open fitting allows natural sound to reach the ear. This sound is supplemented by amplified high frequency sound from the hearing instrument receiver. The open fitting eliminates problems with occlusion, which makes the sound of chewing and one's own voice seem unnaturally loud. When the hearing instrument wearer is in a noisy environment, a closed fitting would be preferred. The closed fitting allows the instrument to have greater control over the sound to reach the ear. The closed fitting also offers increased directivity, noise reduction, and other features which increase intelligibility.
A need, therefore, exists for a hearing instrument which can provide the benefits of open and closed fitting.
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